JPH0249403A - Varistor - Google Patents

Abstract

PURPOSE:To prevent the deterioration with time of varistor voltage and the coefficient of non-linear voltage by a method wherein the electrode, provided in the interior or on the surface of ceramics, is formed with oxide ceramics which attain a superconductive state at a low temperature. CONSTITUTION:The ceramics, having the characteristics of non-linear voltage and an electrode, consisting of oxide ceramics showing superconductive property at a low temperature which will be provided in the interior or on the surface of the ceramics, are provided on the title varistor. The oxide ceramics showing superconductivity at a low temperature is the material consisting of Ln-M-Cu-O oxide ceramics (Ln shows Y, Yb, Er, Ho, Dy and Tm, and M shows Ba, Sr and Ca). As a result, the deterioration with time of varistor voltage and the coefficient of non-linear voltage can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a varistor used, for example, as a surge absorption element, and particularly to a varistor whose electrodes are improved.

(Prior art) Various structures have been used in the past, such as varistors in which electrodes are formed on the surface of ceramics that exhibit voltage nonlinear characteristics, and multilayer varistors that have internal electrodes in addition to the electrodes formed on the surface. things are known. As electrodes of this type of varistor, noble metals such as silver, palladium, or platinum, or alloys thereof are generally used.

(Technical problem to be solved by the invention) However,
The above-mentioned electrode materials are expensive because they are precious metals, making the cost of the varistor high.

In addition, when using electrodes made of silver or palladium,
Migration of the electrode material to the varistor material tends to occur, and as a result, there is a problem in that moisture resistance and the like are not sufficient and short circuits easily occur when an electric field is applied.

By the way, the varistor voltage is determined by the number of grain boundaries existing between the electrodes in the varistor material. Therefore, when manufacturing varistors for low voltage applications, this grain boundary failure must be reduced.

However, as the number of grain boundaries decreases, defects in the varistor material have a greater influence on the electrical characteristics.

Therefore, in the case of low voltage varistors, the migration of silane on the electrodes as described above has become a bigger problem.

For example, in the case of a chip-type varistor, if you are trying to obtain a varistor voltage of 5 to 10 cm, the number of grain boundaries in the thickness direction is 2 to 10 cm.
4, but when electrodes made of silver or palladium were used, sufficient electrical characteristics could not be obtained due to migration.

However, when electrodes made of platinum are used, the above migration is unlikely to occur. However, the introduction of oxygen in the atmosphere into the varistor material during sintering is blocked by the platinum electrode, This also prevents the scattering of the binder mixed in the ceramic material.
The sintered density was not sufficient, making it difficult to obtain a varistor with stable characteristics.

Therefore, an object of the present invention is to provide a varistor equipped with electrodes that can effectively take advantage of the voltage nonlinearity of ceramic varistor materials.

[Means for Solving Technical Problems] The varistor of the present invention is made of a ceramic having voltage nonlinear characteristics and an oxide ceramic that is provided inside or on the ceramic and exhibits superconductivity at low temperatures. The device is characterized by comprising an electrode.

The "oxide ceramics exhibiting superconductivity at low temperatures" used in the present invention are Ln-M-Cu-0-based oxide ceramics (Ln is Y, Yb, or Er).

Ho, Dy, Tm, M consist of Ba, Sr, Ca), more specifically, the general formula: L n M! CLls O? 4 (However, L
n is a lanthanide element selected from the group consisting of Y, Yb, Er, Ho, Dy and Tm; M is Ba, Sr and C;
It is an alkaline earth metal selected from the group consisting of a. )
Alternatively, it refers to a superconducting oxide ceramic represented by the general formula: (LaM)*Cubs (where M is an alkaline earth metal such as Ba, Sr, or Ca).

The present invention is characterized by using an electrode made of an oxide ceramic exhibiting superconductivity as described above, and various known ceramics having voltage nonlinear characteristics can be used. Further, the ceramic having voltage non-linear characteristics may have a single layer structure, or may have a multilayer structure in which layers with different resistivities are alternately laminated.
Its physical structure does not matter.

[Operation and effects of the invention]

The superconducting oxide ceramics mentioned above not only exhibit superconductivity at low or sintered temperatures, but also exhibit 10-
The present inventors focused on such conductivity at room temperature, investigated the possibility of using it as an electrode for a varistor, and through repeated experiments, achieved the present invention. It is something.

In the present invention, since the electrode provided inside or on the surface of the ceramic is made of oxide ceramic that exhibits superconductivity at low temperatures, migration does not occur as in the case of conventional electrodes made of silver or palladium. Therefore, a varistor with excellent moisture resistance etc. can be obtained. In addition, in the conventional example using a platinum electrode, migration was difficult to occur, but the characteristics were unstable because the sintered density of the varistor material was not sufficient. As is clear from the examples,
It is possible to realize a varistor with stable characteristics.

That is, according to the present invention, it is possible to prevent the deterioration of electrical characteristics, that is, the deterioration of the varistor voltage and the voltage nonlinear coefficient α over time, and it is possible to realize a varistor with excellent reliability. Become.

[Explanation of Examples]

Examples of the present invention will be described below.

11 Medical ZnO 97.5 mol%, B l z Os , M n
O.

S b*Os , Crz O@ and Co, O, are each set to 0. The mixture was mixed to have a concentration of 5 mol %, pulverized, an organic binder was added and mixed, and then molded into a disc with a diameter of 10 mm and a wall thickness of 0.5 mm. This molded body was fired in air at 1250″C for 2 hours to obtain a sintered body.

On the other hand, Y@03, BaCOs, and CUO were mixed so that Y, Ba, and Cu were in a molar ratio of 1:2:3.
were mixed, sintered at 950°C, and then pulverized. Fes was added to the pulverized powder to obtain the oxide ceramic paste of the example.

As a comparative example, A, a silver paste obtained by adding Fess to the powder was prepared.

The above two types of pastes were applied to the surface of the sintered body to a size of 6 m in diameter, and baked at a temperature of 600°C.

The characteristics of the obtained varistor are shown in Table 1 below.

Table 1 *1 Voltage nonlinear coefficient *2 Change in varistor voltage immediately after applying a 1500A surge.

*3...Change in varistor voltage after being left for 2000 hours at 60°C and 90% relative temperature.

*4... Change in varistor voltage after being left in the same environment as *3 for 4000 hours.

As is clear from Table 1, the comparative examples using silver electrodes had poor moisture resistance and the varistor voltage tended to decrease over time. Therefore, if used continuously for a long time, there is a possibility of short circuit.

On the other hand, in the varistor of the example, the varistor voltage tends to increase over time, and therefore it can be seen that it can be used more safely.

Note that solder does not adhere to the electrodes made of oxide ceramics in the varistors of the examples, but when attaching lead terminals, a plating layer may be formed with copper or the like.
In this case, it was confirmed that the electrical characteristics did not change even if the plating layer was formed.

Urajoban 1 Zn095.8 mol%, B2O, 1.5 mol%, Mn
1.0 mol% of O, 0.0% of Cot's. 5 mol%, Pr
aO++ 0.2-+-le%, T10. 1.0 mol%
Mix it so that it becomes, add an organic binder, and make 40p
It was molded to a thickness of m.

On the other hand, an electrode paste of a comparative example mainly composed of PT and Ag-Pd and the oxide ceramic paste used in Example 1 were prepared, and these were respectively printed on the surface of the above sheet and laminated one after another. , pressurized and crimped. After each of the obtained laminates was fired in air at a temperature of 900° C. for 3 hours, external electrodes mainly made of silver were baked on both end faces.

The electrical characteristics and aging characteristics of each varistor thus obtained are shown in Table 2 and FIG.

Table 2 This is because a short circuit has occurred due to the four currents.

Furthermore, as is clear from FIG. 1, in varistors using electrodes made of Pt or Ag-Pd, the varistor voltage tends to decrease over time, whereas in the varistor of the example, the varistor voltage It can be seen that there is a tendency to increase over time.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the temporal characteristics of the varistor voltage of Example 2 and Comparative Example.

Claims (1)

[Claims] A varistor comprising: a ceramic having nonlinear voltage characteristics; and an electrode made of an oxide ceramic that is provided inside or on the surface of the ceramic and exhibits superconductivity at low temperatures.